The invention relates to a method and apparatus for the continuous treatment of cellulosic fiber material in three separate vessels, a first vessel for impregnation of the fiber material with digesting liquid, a second vessel for the digestion of the fiber material, and a third vessel for washing of the fiber material.
In the past, the digestion and washing of fiber material in separate vessels has been known (such as shown in U.S. Pat. Nos. 2,870,009, 2,999,785, and 3,298,209), however in such prior art devices the digested chips have been subjected to a significant pressure drop before washing thereof, which usually results in the disintegration of the chips into fiber and makes countercurrent washing thereof very difficult. According to the present invention, the digested chips are transferred from the digesting vessel to the washing vessel without significantly dropping the pressure so that no disintegration of the chips occurs, and countercurrent washing thereof is practical.
There have also been previous provisions for separate vessel impregnation and digestion-washing, such as shown in U.S. Pat. No. 3,802,957 and in copending commonly assigned application Ser. No. 698,125 filed June 21, 1976 by Michael I. Sherman and James R. Prough, and entitled "Two-Stage Digestion with Between Vessel Heating."
In U.S. Pat. No. 3,802,956, the impregnated chips are "blown" into the separate digesting-washing vessel, which results in numerous disadvantages over the apparatus and method of the present invention, and that shown in the above-mentioned copending application, wherein the chips-liquid mixture is sluiced into the digester from the impregnating vessel. Also, according to the present invention (as in the above-mentioned copending application) the transfer loop between the impregnating vessel and the digester is the heating loop, whereby intimate heating and mixing occur resulting in a more uniform final product. The present invention has numerous advantages over that disclosed in the above-mentioned copending application in overall economy and efficiency of treatment. The provision of three separate vessels means that they all can be shop fabricated. In most present installations, because of the size of the digester-washer, it is necessary to erect it in the field since it is too large to ship, resulting in increased costs over shop fabrication thereof. Also, according to the present invention each vessel may be sized to an optimum diameter for the particular operation that will be taking place therein rather than reaching a compromise for one or two vessel systems wherein the diameters have to suit all purposes. Obviously this increases treatment efficiency. This may be facilitated by the provision of two countercurrent washers in parallel. Also, the capacity of existing installations could easily be significantly increased according to the present invention merely by providing a separate washing vessel and utilizing the existing vessel for increased impregnation and/or cooking capacity.
According to the present invention, the method of treating cellulosic fiber material utilizing first, second, and third separate vertical treatment vessels is provided, the method comprising the steps of continuously feeding cellulosic fiber material entrained in liquid into a top portion of the first vessel, establishing a first flow path of cellulosic fiber material entrained in and impregnated with treatment liquid from the bottom portion of the first vessel to a top portion of the second vessel, liquid substantially filling the second vessel, establishing a column of fiber material in the second vessel below the level of liquid in the second vessel, withdrawing liquid from the top portion of the second vessel, establishing a second flow path of the liquid withdrawn from the second vessel back towards the bottom portion of the first vessel, heating the liquid during transport in the second flow path, feeding a portion of the heated liquid flowing in the second flow path into the first path, while feeding the rest of the heated liquid to a bottom portion of the first vessel, withdrawing fiber material from the bottom of the second vessel and feeding it in a third flow path, without significant reduction of the pressure thereof, to the top of the third vessel, effecting countercurrent washing of the fiber material in the third vessel, and withdrawing washed pulp from the bottom of the third vessel. The withdrawal of liquid from the top portion of the second vessel is preferably accomplished without screening, a "stilling well" being provided at the top of the second vessel, and also the stilling well principle is used for withdrawing digesting liquid from the top portion of the third vessel and establishing a fourth flow path of a portion of the liquid withdrawn from the third vessel from the third vessel back toward the bottom portion of the second vessel, and feeding a portion of the liquid flowing in the fourth flow path into the third flow path, while feeding the rest of the liquid in the fourth flow path to a bottom portion of the second vessel. A portion of the liquid withdrawn from the top portion of the third vessel is also subjected to a large pressure reduction so that a portion thereof flashes into steam, and the spent digesting liquid is removed from the pressure reduction area for ultimate recovery or disposal thereof while the flash steam is returned to a steaming vessel utilized for feeding chips to the first (impregnation) vessel. A countercurrent flow of digesting liquid in the first (impregnation) vessel may be provided (displacement impregnation), and withdrawal of liquid from the first vessel to effect a countercurrent treatment may be accomplished without screening, a "stilling well" being provided in an enlarged bottom portion of the vessel. Also, an enlarged bottom portion of the second, digesting, vessel may be provided and a portion of liquid withdrawn from a "stilling well" portion thereof and passed to a flash tank or the like, whereby the temperature and the bottom of the digester is kept low enough so that no degradation of the pulp takes place.
According to the apparatus of the present invention, apparatus for the treatment of cellulosic fiber materials comprises a first vertical treatment vessel, a separate second vertical treatment vessel which is substantially liquid filled, a separate third vertical treatment vessel which is substantially liquid filled, means for feeding cellulosic fiber material entrained in treatment liquid to the top portion of the first vessel, means for establishing a first flow path of cellulosic fiber material entrained in and impregnated with treatment liquid from the bottom portion of the first vessel to the top portion of the second vessel, a column of fiber material being established in the second vessel below the level of liquid in the second vessel, means for establishing the second flow path of liquid withdrawn from the top portion of the second vessel back toward the bottom of the first vessel, means for heating the liquid in the second flow path, means for feeding a portion of the heated liquid in the second flow path into the first flow path while feeding the rest of the heated liquid to the bottom portion of the first vessel (the bottom of the first vessel providing a heating chamber), means for withdrawing fiber material from the bottom of the second vessel and feeding it in a third flow path, without significant reduction of the pressure thereof, to the top of the third vessel, means for effecting countercurrent washing of the fiber material in the third vessel, and means for withdrawing washed pulp from the bottom of the third vessel. Inter-mixing and heating of the liquid and entrained fiber material before the fiber material is fed into the top of the second vessel is accomplished according to the present invention, and an economic, efficient structure results for the digesting and washing of cellulosic fiber material. A fourth vertical treatment vessel may also be provided, the third and fourth vessels being connected in parallel in the third flow path with the bottom of the second vessel, and means for effecting countercurrent washing of fiber material in the fourth vessel being provided. Such an arrangement allows for optimization of the relative diameters in the impregnation, cooking, and washing vessels. Preferably, when liquid is withdrawn from the top portions of the second or third vessels, or when liquid is withdrawn from the first vessel to effect countercurrent treatment in the first vessel, a "stilling well" is provided so that the liquid may be withdrawn without screening.
The primary object of the present invention is to provide a method and apparatus for effecting efficient digestion and washing of cellulosic fiber material to provide a pulp of uniform quality in the most economic manner. This and other objects of the invention will become apparent from an inspection of the detailed description of the invention and from an inspection of the appended claims.